The introduction of the zebra mussel (Dreissena polymorpha) to North America in 1986 has had a dramatic impact on water users throughout the continent. This organism has colonized the entire Great Lakes system, the Mississippi River and associated tributaries and inland waterways throughout North America. Another species of mussel, the quagga mussel (Dreissena bugensis) was introduced in the 1990's and has also spread throughout North America.
Although they are separate species they share a trait which has been devastating to raw water users throughout the continent. Mussel veligers (initial offspring) reach a stage in their maturation wherein they settle and attach themselves to hard surfaces. This is particularly troublesome for industrial water users as service water piping provides an ideal surface for attachment. Resulting problems include reduction of pipes bores and associated cooling capacity, enhanced electro-corrosion of steel and increased maintenance costs associated with the accumulation of shell debris. Since the early 1990's, industry has spent millions of dollars searching for mitigation methods which are reliable, cost effective and safe.
Traditionally, continuous, high-level chlorination via permanent chemical injection facilities has been the most widely implemented treatment methodology for mussel control. The process required that incoming raw water was chlorinated continuously throughout the organism's reproductive period (May-December) thereby preventing colonization. However these methods are encumbered by a number of factors including the high cost of operation of chlorination systems and increased environmental risk due to long extended periods of chemical use and storage.
Canadian Patent Application No. 2,230,582 describes a process for inhibiting the settlement of post-veliger zebra mussels which requires a chlorine dioxide generator to intermittently pump chlorine dioxide into the water system to obtain and maintain a chlorine dioxide concentration of 0.1-0.5 ppm for a period of 10 minutes to 120 minutes. The process is carried out at least 5 days per week each week during the zebra mussel spawning season, typically between April and October. Chlorine dioxide is unstable and potentially explosive and therefore requires the use of an on-site chlorine dioxide generator.
Canadian Patent Application No. 2,091,928 describes the treatment of fresh water to control and prevent infestation by zebra mussels at submerged offshore water intake facilities where water containing chlorine dioxide is continuously or intermittently injected for a treatment period of 1 to 14 days. Experiments carried out to evaluate continuous 24 hour treatment using ClO2 (chlorine dioxide), NaOCl (sodium hypochlorite), KMnO, or NaClO2 (sodium chlorite) were disclosed and only chlorine dioxide was reported to have achieved acceptable mortality rates of 50% at 2 ppm and 90% at 5 ppm, while the other chemicals achieved no mortality or mortality rates of only 10%.
There remains a need for new and improved methods for controlling bivalve growth in the service water systems.